Elucidating the active phases of CoOx films on Au(111) in the CO oxidation reaction

Noble metals supported on reducible oxides, like CoOx and TiOx, exhibit superior activity in many chemical reactions, but the origin of the increased activity is not well understood. To answer this question we studied thin films of CoOx supported on an Au(111) single crystal surface as a model for the CO oxidation reaction. We show that three reaction regimes exist in response to chemical and topographic restructuring of the CoOx catalyst as a function of reactant gas phase CO/O2 stoichiometry and temperature. Under oxygen-lean conditions and moderate temperatures (≤150 °C), partially oxidized films (CoOx<1) containing Co0 were found to be efficient catalysts. In contrast, stoichiometric CoO films containing only Co2+ form carbonates in the presence of CO that poison the reaction below 300 °C. Under oxygen-rich conditions a more oxidized catalyst phase (CoOx>1) forms containing Co3+ species that are effective in a wide temperature range. Resonant photoemission spectroscopy (ResPES) revealed the unique role of Co3+ sites in catalyzing the CO oxidation. Density function theory (DFT) calculations provided deeper insights into the pathway and free energy barriers for the reactions on these oxide phases. These findings in this work highlight the versatility of catalysts and their evolution to form different active phases, both topological and chemically, in response to reaction conditions exposing a new paradigm in the catalyst structure during operation.

• The films are quite thin, in the monolayer range, which means that their properties are affected by the gold substrate.This will also be the case for the catalytic properties, which limits the scope of the results to quite thin cobalt oxide films on Au(111) with the gold being a relevant ingredient.
• A structural characterization with LEED, STM, etc was not performed.It may be the case, that the DFT optimized structure is correct (or at least near to the real structure), but an experimental verification would improve the level of trust.
• In the context of the SESSA simulation discussion it is said that cobalt and gold are immiscible.While this may be correct, it may still happen that thermodynamics drives cobalt atoms into the gold at elevated temperature (oxidation at 200°C).Thus, the real Co coverage may be smaller than the reported one.
• The ResPES experiments were performed to separate Co2+ and Co3+ contributions (some degree of differentiation should actually also possible with the Co 2p levels if the data are measured with reasonably good statistics) The resonances in ResPES have a certain width, which means that there will be a contribution from the other component at both resonance energies (Co2+ and Co3+).It appears that for the Co2+ case, off-resonance intensity is considered, while Co2+ intensity is considered for Co3+ (arrows in Fig. 5d).However, the contribution of Co3+ to the Co2+ spectrum is seemingly not considered.For the reader it may also be a bit surprising that there is such a high intensity at the Co3+ resonance in Fig. 5e.The resonance energies may also be somewhat different for CoO and Co3O4, likewise the linear correction factors.The authors my consider clarifying the description of the spectral Co2+/Co3+ separation a bit.
• Regarding the new paradigm: it appears that this refers to the combined changes of the morphology and the chemical state.Is this really so new?There have been operando TEM studies (Schlögl/Willinger but also others) where the authors report shape and chemical phase changes during a reaction.This also seems to involve both types of change.
• The manuscript does not contain catalytic reactivity studies.Thus, a reactivity comparison of the films is not really possible (beyond what follows from the spectroscopic data) although this could be interesting given that the manuscript deals with catalytic CO oxidation.
The results seem to sound.However, the limited scope of the data, the missing structural characterization and the missing reactivity data lower the relevance somewhat.
Reviewer #2 (Remarks to the Author): This manuscript investigated the topographic restructuring and evolution of CoOx catalysts supported on Au(111) single crystal surfaces in response to reaction conditions.The active sites for CO oxidation were determined by characterization methods and DFT calculations.I would conclude the work is of potential interest to be published on Nat.Commun.However, the following issues regarding the theoretical calculations need to be carefully clarified.
1. Since CoO is antiferromagnetic below Neel temperature, the DFT calculations need to specify the Co atomic magnetic moment, otherwise it is hard to achieve accurate results.
2. The authors do not provide a convincing explanation for the contradictory conclusions between the experiments and the DFT calculations of carbonate formation on the stoichiometric CoO films.
3. The authors proposed that Co3+ sites have a unique role in CO oxidation, but lacked a reasonable analysis to elucidate the chemical nature.Also, the authors considered that partially oxidized films (CoOx<1) containing Co0 are efficient catalysts, so a comparison of these two sites is necessary.
4. There have been many similar studies on the structural evolution of CoOx catalysts during CO oxidation, and the authors should compare this work with other studies.
Reviewer #3 (Remarks to the Author): The manuscript by Chen et al. (NCOMMS-23-25699-T) reports the existence of three different CO oxidation reaction regimes which depend on the chemical state of the catalyst which, in turn, depends on the gas phase CO/O2 stoichiometry.The authors employed Ambient Pressure XPS (APXPS) and Resonant Photoemission Spectroscopy (ResPES) to monitor the oxidation state of model cobalt catalyst under reaction conditions.The key result of this study is the observation of Co3+ ions associated with the formation of Co3O4 phase and their involvement in CO oxidation.Note that detection of small amounts of Co3+ based on the core level spectra is extremely difficult due to its complex shape.The reported results are particularly important for cobalt-based catalysts often used in combination with supported noble metal nanoparticles, where redox interactions play a crucial role in the catalyst reactivity and selectivity.Monitoring and quantitative analysis of the oxidation state of cobalt-based catalyst allows to obtain comprehensive insights into catalyst activity which is controlled by redox interactions.
The text of the manuscript is clearly written.The literature review is comprehensive.The experimental data are of good quality and should be easily reproduced.The data were analyzed and interpreted carefully and are presented in sufficient detail.However, I have concerns about the calibration of RER parameter (see questions below).The experimental evidence for the conclusions is strong.(The evaluation of theoretical study is out of my expertise).I recommend to accept the manuscript for publication in Nature Communications after the authors address the question listed below.
1) Figure 3. Authors should comment on the appearance of sharp peak in C 1s region (around 283.0 eV) obtained from 1 ML CoO0.25 catalyst under exposure to CO at and above 100 C.This could point to formation of cobalt carbides due to additional reaction pathway, e.g.via CO disproportionation.
2) Figure 4.The signal in the Co 2p region is unusually low at 375 C under exposure to CO.The authors explain this by combined oxide reduction upon decomposition of carbonates and CoO deweting and formation of CoO clusters.Did the authors verified such a scenario by simulation in SESSA?Can the authors rule out desorption of cobalt carbonyl species?
3) Lines 283-294.The authors determined RER of stoichiometric Co3O4 to be 0.67.This value is very different to the value determined earlier on well-ordered stoichiometric Co3O4(111) films (RER=0.9) in Ref.

Response to the Reviewer 1:
Reviewer #1 (Remarks to the Author): The promotion of CO oxidation by cobalt oxide is discussed in a number of publications, but a comprehensive picture has not yet been obtained.The submitted manuscript targets this topic, addressing especially the catalyst's stoichiometry and morphological/chemical transitions.This is a meaningful approach, but there are some remarks.
Reply: We thank the reviewer for careful reading of the manuscript and for raising the constructive comments.Our point-by-point responses are listed below.
• The films are quite thin, in the monolayer range, which means that their properties are affected by the gold substrate.This will also be the case for the catalytic properties, which limits the scope of the results to quite thin cobalt oxide films on Au(111) with the gold being a relevant ingredient.

Reply:
The reviewer raises a good and important point.The Au(111) was chosen as substrate for the growth of oxide films because, unlike Pt [4,5] or other noble metals [6][7][8] that are active in the CO oxidation reaction, the Au (111) surface does not participate in the CO oxidation reaction because of its weak adsorption of CO and because it cannot dissociate O2 efficiently.However, as the reviewer points out, Au can modify the reactivity of the monolayer CoO film.We studied this question with the help of DFT calculations and found that the reaction energy of the CO oxidation reaction on a second CoO layer is higher than that of the first layer by ~0.4 eV.This topic, together with a comparison between Au and Pt substrates is discussed in detail in a paper now in preparation.
Here the focus is on the reactivity of different Co oxidation states and on the configurational changes that occur in response to the reactant gas composition.Following the reviewer's comment, we have included a mention of the influence of Au in the revised version of the manuscript.

Proposed changes (highlighted in yellow):
"Although the Au substrate does not participate in the CO oxidation reaction because of the weak adsorption of CO and inefficient dissociation of O2, it can still modify the reactivity of the first CoOx layer in contact with the substrate.Our most recent results suggest that Au does indeed modify slightly the reactivity of the single monolayer CoO film, but has a lessened effect on the reactivity of the second layer, which remain slightly lower than that of the first." • A structural characterization with LEED, STM, etc was not performed.It may be the case, that the DFT optimized structure is correct (or at least near to the real structure), but an experimental verification would improve the level of trust.
Reply: We agree with the reviewer.The Au-CoOx system has been studied using LEED, STM and other surface science techniques by the groups of Prof. Lauritsen in Denmark [9][10][11][12] and Prof. E. I.
Altman in the United States [13], whose results on the structure of the oxide layers are in line with our DFT calculations.In our previous STM results with similar systems, CoOx/Pt(111) [5], and FeOx/Pt(111) [14][15][16][17][18][19][20], all prepared in similar conditions, we found similar structures, also in line with our DFT calculations.This makes us confident in the calculated DFT structures of CoOx/Au(111).We have included the following in the revised version of the manuscript."partition functions.[21] … The structure of ultrathin CoOx films on the hexagonal (111) surfaces of Au and Pt by STM has been studied extensively in the past.[9][10][11][12] In all cases the films form hexagonal lattices with slightly different unit cells from that of the metal substrate leading to formation of Moire patterns.Our previous STM and DFT studies of CoOx/Pt(111) [5] and FeOx/Pt(111) [14][15][16][17][18][19][20]22], and the more recent work of Zeuthen et al. on FeOx/Pd(111) [23], again report similar structures.Here we follow the crystallographic structure of the CoOx/Au(111) films and their change during reaction by theoretical modeling using the same DFT approach.Details of the structural models are described in the SI (Fig. S3).
• In the context of the SESSA simulation discussion it is said that cobalt and gold are immiscible.

While this may be correct, it may still happen that thermodynamics drives cobalt atoms into the gold at elevated temperature (oxidation at 200°C). Thus, the real Co coverage may be smaller than the reported one.
Reply: As stated in the manuscript, heating under reducing conditions accelerates the dewetting of the CoOx, which is related to the week affinity for formation of Co-Au bonds.The immiscibility of Co and Au in the temperature conditions of our experiment was also manifested in our unsuccessful attempts to synthesize Au-Co nanoparticle alloys by intimately mixing AuCl3 and Co(acac)2 molecular precursors.This is strong evidence of the difficulty to form a stable Co-Au alloy under our conditions.The immiscibility between Au and Co is also in line with the Au-Co bimetallic phase diagram [24], that exhibits no significant Co fraction in Au for temperatures below 500 °C.

• The ResPES experiments were performed to separate Co2+ and Co3+ contributions (some degree of differentiation should actually also be possible with the Co 2p levels if the data are measured with reasonably good statistics)
Reply: Yes, the reviewer is right.Both we [5] and others [25][26][27] have fitted the Co XPS peaks that are composed of overlapping Co peaks in various oxidation states (0, +1, +2, and +3) and could obtain a good fit with the experimental peaks by convolution of these components.The fits provided a possible answer to the amount of each Co n+ species.However, we believe that this is not sufficiently convincing.After all, with sufficient peaks one can fit anything.ResPES, on the other hand, separates unambiguously these oxidation states.

• The resonances in ResPES have a certain width, which means that there will be a contribution from the other component at both resonance energies (Co2+ and Co3+). It appears that for the Co2+ case, off-resonance intensity is considered, while Co2+ intensity is considered for Co3+
(arrows in Fig. 5d).However, the contribution of Co3+ to the Co2+ spectrum is seemingly not considered.For the reader it may also be a bit surprising that there is such a high intensity at the Co3+ resonance in Fig. 5e.The resonance energies may also be somewhat different for CoO and Co3O4, likewise the linear correction factors.The authors may consider clarifying the description of the spectral Co2+/Co3+ separation a bit.

Reply:
The reference oxides used here are CoO with only 2+, and stoichiometric Co3O4 with a mixture of 2+ and 3+.The fact that both the green and the blue spectra (new version of Fig. 5 shown in below Fig. R1) acquired at the two different resonant energies in Fig. 5d have very similar shapes is because both originate from the same Co 2+ state.The lower intensity of blue curve is due to the slightly off resonance value of the 781.2 eV photon energy used.It also tells us that there is no, or very little, Co 3+ in this sample.So, there is nothing to be subtracted from the spectra.The fact that we used the color and photon energy for the Co 3+ resonance in the previous version is what confused the reviewer.That is now corrected in the edited version of the manuscript (Fig. R1).
In contrast, on Co3O4 (Fig. 5e) we have two distinct lines for the spectra acquired at the two resonant energies, one for the 3+ state with the prominent peak in the red curve near 1 eV BE.The other spectrum (green line) corresponds to Co 2+ , and has a similar shape as that from the CoO sample in Fig. 5d, as expected.We thank the reviewer for revealing our poor choice of colors and explanation.We have corrected this and also modified the figure to avoid this confusion."…Fig.5b-c.For the CoO, only Co 2+ species are present, with a resonant photon energy of hω = 779.8eV, as shown in the heat map.The VB d-states of Co 2+ with peaks at ~5 eV and ~10 eV, are strongly enhanced at this photon energy (green trace in (Fig. 5d).For hω =781.2eV,slightly off resonance, the spectrum is similar as expected, but less intense (blue trace), and for hω = 772 eV (far from resonance), the VB spectrum (light blue trace) is dominated by the Au substrate.For Co3O4 (Fig. 5c) the resonant photon energy for the Co 3+ site is 781.2 eV, as shown by the maximum in the heat map.The VB spectrum at this photon energy shows several Co 3+ d-band peaks: a sharp one at 1.0 eV, and others around 10 eV, and 5.0 eV (Fig. 5e).The VB…" • Regarding the new paradigm: it appears that this refers to the combined changes of the morphology and the chemical state.Is this really so new?There have been operando TEM studies (Schlögl/Willinger but also others) where the authors report shape and chemical phase changes during a reaction.This also seems to involve both types of change.

Reply:
The reviewer is correct that changes in catalyst structure driven by reactants and products is not a novelty, as we have also shown in the past.However, old beliefs are persistent and it is still normally assumed that catalysts do not change.Here we add an example of a double restructuring: chemical and structural, both strongly correlated, driven by reactants and products.
• The manuscript does not contain catalytic reactivity studies.Thus, a reactivity comparison of the films is not really possible (beyond what follows from the spectroscopic data) although this could be interesting given that the manuscript deals with catalytic CO oxidation.
Reply: Correct.We can only conclude that the activity increases by the presence of Co 3+ with the novelty that this can now be unambiguously related to these oxidized Co species, as proven by our in operando spectroscopic results.
• The results seem to sound.However, the limited scope of the data, the missing structural characterization and the missing reactivity data lower the relevance somewhat.

Reply:
We really appreciated this positive comment.Although, suffering from limited access to the in-situ techniques, such as STM and other surface sensitive methods, which can provide more operando observations about CoOx-Au model catalyst during the CO oxidation reaction, we successfully followed changes of the chemical states and morphological changes of CoOx by APXPS, which we further supported by advanced DFT calculations.

Reviewer #2 (Remarks to the Author): • This manuscript investigated the topographic restructuring and evolution of CoOx catalysts supported on Au(111) single crystal surfaces in response to reaction conditions. The active sites for CO oxidation were determined by characterization methods and DFT calculations. I would
conclude the work is of potential interest to be published on Nat.Commun.However, the following issues regarding the theoretical calculations need to be carefully clarified.

Reply:
We thank the reviewer for his/her careful reading of the manuscript and for raising constructive comments.Our point-by-point responses are listed below.

• Since CoO is antiferromagnetic below Neel temperature, the DFT calculations need to specify the Co atomic magnetic moment, otherwise it is hard to achieve accurate results.
Reply: Indeed, we have accounted for the spin state of Co and its antiferromagnetism in our calculations.In the updated manuscript and SI, we have added details of the spin states of the Co cations and an extra line in the methods section of the manuscript referring to them.Line 104: "…allowed to relax.Following our previous work on CoOx<1/Pt, a row-wise antiferromagnetic structure was used for all CoOx<1 models.In order from top-left to bottom-right in the outlined unit cell (Fig. S3a), the magnetic moments of the Co atoms were found to be -2.32,-1.99, 2.49, and 2.49 μB.For these structures…" Line 115: "… Following previous studies of thin CoO films, a row-wise antiferromagnetic structure was used for all CoO models.The absolute magnetic moment of the Co cations in the films with 3.00 Å and 3.25 Å lattice spacing were found to be 2.36 and 2.57 |μB|, respectively.For these structures…" Line 132: "For Co3O4, the antiferromagnetic alignment of Co 2+ cations in tetrahedral sites was maintained in all calculations.The magnetic moment of the outermost Co cation was found to be -2.85 μB, while the magnetic moments of the three Co cations in the subsurface were found to be -0.09,1.85, and 1.84 μB.For CoO2, in the order of top-left to bottom-right in the outlined unit cell (Fig. S3d), the magnetic moments of the Co atoms were found to be 0.00, 0.00, 1.26, and 1.26 μB."

The authors do not provide a convincing explanation for the contradictory conclusions between the experiments and the DFT calculations of carbonate formation on the stoichiometric CoO films.
Reply: Although we were unable to find thermodynamically stable configurations of carbonate groups on the terraces of CoO films, the DFT-optimized geometry of both a single bidentate carbonate and the CoCO3 film suggest that carbonates are metastable and induce restructuring and dewetting of the oxide film.A more complete study of this process would require much larger models.Beyond the formation of an unstable CoCO3, we found that formation of carbonates on the CoO/Au(111) film with a 3.00 Å Co-Co spacing is more exothermic than that over the film with a 3.25 Å Co-Co spacing and induces a dewetting restructuring of CoO, where one interfacial Co has moved above O.We have added additional discussion to the description of the CoO reactivity.

Proposed changes:
Page 15 of the manuscript: Line 375: "… (Fig. S7), indicating that, if formed, they should be unstable unless under a high CO2 partial pressure ."Line 387: "…reported for CoO films on Pt(111).We note that the formation of carbonates on the CoO film with 3.00 Å Co-Co spacing also induced a dewetting reconstruction of interfacial Co, where Co detaches from the Au substrate and moves above the surface-bound O (Fig. S7f).This restructuring also supports a more complex structural transformation of CoO upon the formation of carbonate groups."

The authors proposed that Co3+ sites have a unique role in CO oxidation, but lacked a
reasonable analysis to elucidate the chemical nature.Also, the authors considered that partially oxidized films (CoOx<1) containing Co0 are efficient catalysts, so a comparison of these two sites is necessary.

Reply:
We believe that the over-oxidized Co 3+ species are responsible for the reactivity as they are readily reduced in the reaction between CO and surface O, which is reflected in the change of the Co cations' magnetic moments after reaction.The reduction of Co 2+ on the other hand is much more difficult.
In addition, although the sub-oxidized films are also somewhat reactive towards CO, they are unlikely to be responsible for the reactivity of CoOx/Au as the phase was not observed in O-rich CO oxidation environments and is unlikely to be a contributor to reactivity.We have added both explanations to the manuscript.

Proposed changes:
To Page 16 of the manuscript: Line 406: "…more reactive.Further, under steady state CO oxidation, the sub oxidized phase was not observed."Line 410: "… (Fig. 7b).The formation of the O vacancy is linked to the reduction of subsurface Co. Upon the formation of the O vacancy, the magnetic moments of the two subsurface Co surrounding the O vacancy shifted from 1.85 and 1.84 μB to 1.90 μB, while the magnetic moment of the third Co changed from -0.09 to 2.59 μB, indicating reduction.Note that the electronic energy of reaction is -1.80 eV relative to CO gas, which is much more exothermic than the reaction over CoO (Fig. S7).The CO3 2-group…" 4.There have been many similar studies on the structural evolution of CoOx catalysts during CO oxidation, and the authors should compare this work with other studies.
Reply: Indeed, the stability of the CoOx phases have been studied extensively by the group of Lauritsen, where they proposed that the most stable phase is CoO2 under an oxidative environment.
We have updated the text to refer to their findings and support our choice of highly oxidized phases.

Proposed changes:
Page 15 of the manuscript: Line 395: "… a CoO2 film (Fig. S9).The structure of CoOx films supported on Au and Pt under oxidative conditions have been extensively characterized.For CoOx/Au, it has been proposed that the O-rich CoO2 phase is the most stable configuration under O2.[11,28,29]  Reply: This reviewer raises a good point.We verify the dewetting process of reduced CoOx clusters and rule out the formation of the carbonyl species as the reappearance of Co2p peak after re-oxidizing this reduced CoOx clusters on Au (111) under 100 mTorr of O2 at ~350 o C (Fig. R2).
The explanation of this dewetting process is to be found in the phase diagram of the Au-Co binary system [24] which, as mentioned before, shows that Co and Au are immiscible.Thus, reduction of Co 2+ to Co 0 leads to deweting by diffusion of the Co 0 and formation of 3-D Co clusters.A similar behavior has been reported by Parkinson et al. [31] showing that well-dispersed Ir adatoms on a magnetite surface merged into large clusters after thermal annealing at higher temperature.Reply: We thank the reviewer for pointing this out.When we preparing this draft, our group members also discussed on this discrepancy.We did the ResPES on the reference samples, which were confirmed to be CoO and Co3O4 by the Co2p XP spectra.The difference in RER may originate from the Au substrate's contribution.The thickness of the CoOx film supported on the Ir(100) is ~6.0 nm (data from Ref. 27) while the thickness of the CoOx film supported on the Au(111) is ~0.3 nm in this manuscript.Therefore, we can still observe some peaks on the offresonance Valence Band spectra that correspond to d-levels of the Au substrate (Fig. R3a).
However, the contributions from the substrate's d-levels in Ref. 27 is negligible (Fig. R3b).When we did the calculation of RER value on the nonstoichiometric CoOx ultrathin layer on Au, we consistently subtracted the contributions from the Au d-levels.Therefore, the stoichiometry of CoOx we get under the CO oxidation reaction is valid.Line 212: "… at the higher temperature) (Fig. 3b).This is …" Line 213: "…Co 0 peak at 778 eV (Fig. 3(b,c))" changes to "Co 0 peak at 778 eV (Fig. 3a,3c)" Line 223: "…and (b) O 1s core level regions" changes to "…and (c) O 1s core level regions"

27 .
It is hard to believe that stoichiometric compound could give such different values.The authors should provide evidence that their Co3O4 sample used for calibrations of ResPES has a structure and stoichiometry of Co3O4.Minor a) Lines 203.Check the labeling of panels in Figure 3. O 1s and Co 2p are shown in (c) and (a), respectively Proposed changes: Page 5 and line 121 of the Main text:

Figure. R1
Figure.R1 New version of the Fig.5 in the manuscript To determine the linear correction factor (y), the D(Co 2+ ) and D(Co 3+ ) values from the Co3O4 film are needed.These values correspond to the length of the arrows at 5 eV BE and at 1 eV BE in Fig.5e.In this manner, the contribution of the d levels from Au the substrate is subtracted to get the D(Co 2+ ), while the contribution of both Co 2+ and Au are subtracted in the arrow at 1 eV BE to get D(Co 3+ ).
Proposed changes: To Page 5 and line 110 of the Main text: "… surface redox properties of CoOx.[10] CoO's row-wise antiferromagnetic state was maintained in all calculations; more details regarding the spin state of Co cations are provided in the SI.Structural relaxation…" To Page 5 of the SI:

Reviewer # 3 (
Following our…" Remarks to the Author):The manuscript byChen et  al. (NCOMMS-23-25699-T) reports the existence of three different CO oxidation reaction regimes which depend on the chemical state of the catalyst which, in turn, depends on the gas phase CO/O2 stoichiometry.The authors employed Ambient Pressure XPS (APXPS) and Resonant Photoemission Spectroscopy (ResPES) to monitor the oxidation state of model cobalt catalyst under reaction conditions.The key result of this study is the observation of Co3+ ions associated with the formation of Co3O4 phase and their involvement in CO oxidation.Note that detection of small amounts of Co3+ based on the core level spectra is extremely difficult due to its complex shape.The reported results are particularly important for cobalt-based catalysts often used in combination with supported noble metal nanoparticles, where redox interactions play a crucial role in the catalyst reactivity and selectivity.Monitoring and quantitative analysis of the oxidation state of cobalt-based catalyst allows to obtain comprehensive insights into catalyst activity which is controlled by redox interactions.The text of the manuscript is clearly written.The literature review is comprehensive.The experimental data are of good quality and should be easily reproduced.The data were analyzed and interpreted carefully and are presented in sufficient detail.However, I have concerns about the calibration of RER parameter (see questions below).The experimental evidence for the conclusions is strong.(The evaluation of theoretical study is out of my expertise).Reply: We really thank the reviewer for positive comments and very important questions.Our point-by-point responses are listed below.I recommend to accept the manuscript for publication in Nature Communications after the authors address the question listed below.1) Figure 3. Authors should comment on the appearance of sharp peak in C 1s region (around 283.0 eV) obtained from 1 ML CoO0.25 catalyst under exposure to CO at and above 100 C.This could point to formation of cobalt carbides due to additional reaction pathway, e.g.via CO disproportionation.Reply: We agree with the reviewer's comment that the sharp peak at 283.0 eV may originate from the CO disproportionation on the Co 0 sites at elevated temperature.However, this CO disproportionation reaction (2CO → C+ CO2) would not contribute to the decrease of lattice oxygen (Fig.3c, O1s spectra) when the reaction temperature raised from 100 o C to 150 o C. Therefore, the reaction between CO and lattice oxygen of CoO0.25 is favorable at this temperature.Following the reviewer's comment, we have included a mention of this 283.eVpeak in the revised version of the manuscript.Proposed changes: To Page 9 and line 216 of the Main text: "The new small peak at 283.0 eV originates from cobalt carbides(CoCx), suggesting CO dissociation at Co 0 sites at elevated temperature.[30]Raising the …" 2) Figure 4.The signal in the Co 2p region is unusually low at 375 C under exposure to CO.The authors explain this by combined oxide reduction upon decomposition of carbonates and CoO deweting and formation of CoO clusters.Did the authors verified such a scenario by simulation in SESSA?Can the authors rule out desorption of cobalt carbonyl species?

3 )
Lines 283-294.The authors determined RER of stoichiometric Co3O4 to be 0.67.This value is very different to the value determined earlier on well-ordered stoichiometric Co3O4 (111) films (RER=0.9) in Ref. 27.It is hard to believe that stoichiometric compound could give such different values.The authors should provide evidence that their Co3O4 sample used for calibrations of ResPES has a structure and stoichiometry of Co3O4.

Figure. R3
Figure.R3 ResPES spectra of (a) ~0.3 nm thick CoO x /Au(111) from our draft and (b) ~6.0 nm thick CoO x /Ir(100) from Ref.27.Note that, the color representing different Resonance spectra in above two panels.